This invention relates to a method for inspecting the internal structure of composite materials and particularly, a method for determining the length, number and position of cracks within each ply of a composite material, and also a method for determining the orientation of the fibers contained within each ply of the material.
This invention is particularly useful in inspecting composite materials used in aircraft structures. Since all aircraft are required to be capable of sustaining a specific amount of damage over a given time of unrepaired service, there is a need to track or monitor the actual amount of damage accumulated in flight critical components of the airframe structure.
With regard to metallic components, the inspection of actual damage is a relatively straightforward task involving monitoring the growth of surface connected cracks. With regard to composite structures, a different inspection or design approach was taken. Since the damage mechanism in composite materials is in the growth of a number or the density of small cracks within the high stress regions of the structure, and since these cracks lie between and beneath many layers or plys of the composite material, early non-destructive inspection methods were unavailable, and, therefore, aircraft designs using composite material were very conservative. Over time, confidence in the use of these material has grown, and the safety factors imposed on these materials have been significantly reduced. There is currently a renewed emphasis on a requirement to inspect the actual damage accumulation within composite materials in order to avoid the failure of critical components of an airframe structure and possible aircraft loss due to unexpected material failures.
One prior art technique for inspecting composite materials for cracks utilizes a radiographically opaque penetrant tetrebromo-ethane (TBE) to enhance the image of surface connected cracks on X-ray radiographs. This technique provides a clear indication of very small flaws; however, all of the flaws detected by this method must be surface connected in order for the penetrating fluid to find its way into the flaw, and also, TBE is toxic to humans and can cause stress-corrosion of aluminum structures. The usefulness of this technique is therefore limited, particularly, in operational aircraft where the cracks or flaws to be detected are almost always located beneath non-cracking surface layers.
A second defect or flaw detecting method uses an ultrasonic beam and attempts to determine or infer the presence and the number of small cracks within the material by the attenuation of a normally incident acoustic beam as it traverses the specimen under examination. With this method, however, the presence of a substantial number of small cracks has very little effect on the attenuation of an ultrasonic wave. Indeed, changes in surface roughness, volume fraction of reinforcing fibers, and the like, have a far larger effect on the measured attenuation of the acoustic wave than do numerous small flaws. Also, this technique does not provide any indication of the depth of the cracks within the structure or the orientation of these cracks or flaws.